This is the largest one I know of: http://icofy-blog.de/en/icofy-base/
The Millionbase 2.2 is a little smaller (PGN format): http://www.top-5000.nl/pgn.htm
Millbase is another slightly smaller database: http://katar.weebly.com/2/post/2013/08/millbase-for-scid-update3.html
I just downloaded SCID v PC. Where can I find the biggest database of games to download for it?